Coherentism holds that beliefs are justified by belonging to coherent belief sets. There are many possible coherent beliefs sets, however, and coherentism provides no way of deciding between them. Fictional worlds such as Narnia, the Matrix, and the Discworld are as coherent (or at least could be made as coherent) as the actual world. If coherence is the standard of justification, therefore, then we are as justified in believing in the Discworld as we are in believing in Earth, so long as we are willing to make the necessary adjustments to our other beliefs. This, though, is absurd.
Moreover, many of these belief sets contradict each other; there are coherent belief sets that contain the belief that the world is round, and there are other equally coherent belief sets that contain the belief that the world is flat. In order to decide whether the world is round or flat, therefore, I must use some other standard of justification than coherence. In fact, for every belief there is a coherent belief set that contains it, and so coherentism fails to recommend any belief over any other. It can’t help us at all in deciding what to believe.